The reaction pd $\rightarrow ^{3}$He $\eta$ at 200 MeV excess energy

Waters, M.

Book/Report

FZJ-2018-06688

The reaction pd $\rightarrow ^{3}$He $\eta$ at 200 MeV excess energy

Waters, M. (Corresponding author)FZJ*

1994
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag Jülich

Jülich : Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag, Berichte des Forschungszentrums Jülich 2917, III, 109 p. (1994)

Please use a persistent id in citations: http://hdl.handle.net/2128/20163

Report No.: Juel-2917

Abstract: The PROMICE detector - a subset of the future WASA 4$\pi$-detector - at the CELSIUS storage ring in Uppsala has successfully been used to study the productionof $\eta$ mesons in the reaction d(p, $^{3}$He)$\eta$ at beam energies of T$_{p}$=1250 and 1276 MeV via both invariant mass determination of the two photons from the $\eta$ decay [35] and the detection of the recoil $^{3}$He nucleus. The double-charged $^{3}$He particles were identified from the dE/dx information of the forward scintillator counters. On a broad background from multi-pion production, a clear $\eta$ signal was observed in the missing mass distribution calculated from the forward detector tracks. Center-of-mass angular distributions have been extracted, and those events corresponding to missing mass values outside the range of the $\eta$ missing mass peak have been used to interpolate the structure of the background in the "true" $\eta$ sample. Extensive Monte Carlo simulations have been performed in order to calculate the total detection efficiency. The final angular distributions obtained after folding with the efficiency are strongly forward peaked, and Legendre fits indicate that at $\sim$ 200MeV excess energy s-wave is suppressed compared to p-wave or higher partial wave production. This is reasonable since the rneasurements were carried out dose to a system where a P$_{11}$ N*(1710) resonance with a width of $\sim$ 110 MeV is embedded in a nucleus {dN*}; the threshold for the reaction pd $\rightarrow$ dN* (on-shell excitation of the oenter of the resonance) is at 1.3 GeV beam energy, whereas the threshold for the on-shell production of the S$_{11}$ N*(1535) with a width of $\sim$ 150 MeV is at a beam energy of 990 MeV in this reaction. The measuremeats at the two beam energies are consistent in a way that at the slightly higher energy also increasing contributions from higher partial waves were found. Additional rneasurernents of angular distributions at other beam energies would help to verify this resonance behaviour. In order to clearify the importance ofresonances for $\eta$-production dose to threshold reactions involving three particles in the exit channel should also be investigated since Dalitz plots can reveal information about intermediate resonance states. Furthermore the production of $\eta$ mesons ort targets with A>2 is of interest because an enhancement in the excitation function due to "velocity matching" only occurs with the deuteron as target. With increasing number of participating nucleons multi-step (cascade) processes will play a role in the meson production since they allow for an efficient sharing of the high momentum transfer. The WASA 4$\pi$-detector will in the future enable exclusive measurements of the reactions discussed here. This will improve the efficiency for studies of the intermediate Formation of resonances, their role in the nuclear wave function and their irnportance for bound nuclear systems.

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